Fuel tanks in aircraft are commonly located within the wing structure. Many tubings, for example, hydraulic fluid conduit and fuel supply lines, extend through the fuel tank. The tubing must be supported within the fuel tank. Generally, a support bracket and clamp extend from the fuel tank wall to support the tubing.
During flight, the aircraft wings experience significant vertical displacement. The aircraft structure also experiences high vibration levels, temperature variations, aerodynamic buffeting and structural flexure. Frequently, the tubing is displaced relative to the tank wall supporting the tubing. When there is movement of the fuel tank wall relative to the tubing, the tubing must be permitted to move with respect to the wall to prevent breaking the wall, the tubing or both. In present aircraft, a wear sleeve extends around the tubing, a clamp extends around the wear sleeve, and the clamp is coupled to the fuel tank wall. The clamp is loosely coupled around the wear sleeve so that when the tubing moves relative to the tank wall, the wear sleeve moves axially within the clamp.
Because the tubing is within the fuel tank, the tubing is surrounded by liquid fuel or fuel vapors. If electrical arcing within a fuel tank occurs, particularly in the vapor, a destructive explosion is possible. It is important to minimize or prevent electrical arcing within the fuel tank to ensure that such an explosion does not occur. In older aircraft, because both the tank wall and tubing were metal, electrical arcing could be avoided by grounding them together. In recently designed aircraft, because the tubing is metal and the tank is not metal, electrical arcing between the tubing and the fuel tank wall can occur, even though the fuel tank and tubing are grounded together.
U.S. Pat. No. 4,630,789, to Rosenberg, incorporated herein by reference, teaches that arcing between metal tubing and adjacent structure may be eliminated by coating the tubing with a dielectric insulator along its entire length of interest. While such a dielectric coating is useful to prevent electrical arcing, no provision is made for supporting the tubing within a fuel tank and permitting relative motion between the support assembly and the tubing.
Insulating the tubing from the support clamp using heat-shrink tubing placed over the conduit has been described in U.S. Pat. No. 4,654,747, to Covey, incorporated herein by reference. The heat-shrink tubing acts as the wear sleeve and also provides dielectric insulation between the fuel tank, clamp and tubing. One disadvantage of using heat-shrink tubing is the difficulty in installing or removing the tubing. The heat-shrink tubing must be inserted over the end of the metal tubing, moved to the proper position and then heated. If the insulating layer cracks or otherwise degrades, the metal tubing must be cut out of the aircraft wing and replaced in order to remove and reinstall heat-shrink tubing. This is very expensive and time-consuming.
A further disadvantage of using heat-shrink tubing is that the preferred physical properties required of a wear sleeve are difficult to provide in the material used for heat-shrink tubing. The clamp must be a relatively snug fit around the wear sleeve to prevent radial movement of the tubing but be sufficiently loose to permit axial movement. If the wear sleeve is too small, the tubing will bounce within the clamp causing damage; if it is too large, then the clamp does not fit loosely enough to permit movement of the tubing, and the tubing or tank wall may be broken. Ideally, the inside diameter of the clamp around the wear sleeve is a predetermined constant value and the outside diameter of the wear sleeve is a predetermined constant value. Controlling the outside diameter of heat-shrink tubing is extremely difficult as it depends upon the amount of heat applied, the time of the heat applied and many other factors. Frequently, the outside diameter of a single heat-shrink wear sleeve may vary along the axial length, causing the clamp to stick at some locations. The outside diameter of the heat shrink tubing will also vary from wear sleeve to wear sleeve.